funny... just as I made the first post... I happened to get a pic that I needed to use that same method. It was RGB... used the blue channel and got the details of his hair isolated... had to back in with brush to fill in the black to a solid in some areas and touch up the right side where a shadow made it hard to get details seperated from bg. You can see before and after in pics. I even applied a slight blur to the mask for softer edges in the final with more touch up in the details on the hair.
Ohh... I could probably soften up a few areas but I don't worry about that in my workflow. This pic was pulled into InDesign for advertising use. It has clipping path on it that is not real tight (because of issues with too complex paths and our imagers) and so I have gotten used to feathering objects with a clipping path. So this pic will probably get a 1-2pt feather all around which helps it to blend into my backgrounds better.